Generally, the Global Positioning System (GPS) provides basically two fundamental quantities from each available GPS satellite, e.g., space vehicle (SV). These two fundamental quantities include a pseudorange measurement and SV position. While GPS, itself, is commonly thought of as a position determination system, the pseudorange measurements are actually processed to compute delta positions, i.e., a change or difference in one position with respect to a second position. Such computations are made after receipt of GPS signals from one or more GPS SVs by one or more GPS antenna/receiver sets. The two positions may be, for example, a current position with respect to a previous position, a position of one antenna with respect to another antenna at one particular time (or relative position therebetween), or a current relative position with respect to a previous relative position.
As part of the GPS, each SV continuously transmits a navigation signal, i.e., SV position signal, containing navigation message data such as, for example, time of transmission, satellite clock data, and ephemeris data. The navigation signal is broadcast over two separate carrier signals, denoted as L.sub.1 and L.sub.2, each of which is modulated by a separate pseudorandom digital code that is unique to the SV from which it is transmitted. For conventional GPS navigation, a GPS receiver typically tracks at least four SV's, establishing synchronism with the SVs' transmitted navigation signal by way of a local clock at the receiver, and recovers the navigation message data.
GPS receivers typically derive two types of measurements from the received GPS signals, referred to as "code measurements" and "carrier measurements." For example, the pseudorandom digital code signals recovered by the GPS receiver can be used to provide code measurements including a measure of the distance to each SV, i.e., pseudorange measurement. This is not necessarily the same as actual range to the SV because of the lack of time clock synchronism between the satellite and the GPS receiver, which can be virtually eliminated by using multiple SV pseudorange measurements to correct for lack of clock synchronism. Further, in contrast to using pseudorandom digital code signals to provide pseudorange measurements, similar measurements may also be recovered using measurements of carrier phase made by the GPS receiver which typically provide for more accurate pseudorange measurements, or by other methods such as carrier-smoothed code-based pseudorange. In the case of carrier phase based measurements, accurate knowledge of the phase within a single wavelength is available, however, an unknown integral number of carrier signal wavelengths, the phase ambiguity, between a GPS SV and GPS receiver antenna exists, and must be resolved. The resolution of the ambiguity, including the resolution of any clock error between GPS SVs and GPS receivers, is not addressed herein and is assumed to be corrected by one of any number of methods for correcting such ambiguity available in the art.
Typical GPS receivers for tracking a GPS satellite generally require synchronization with and demodulation of the carrier signal and codes from the GPS signals received. In most designs, a correlation process establishes carrier and code tracking loops that align selected GPS carrier and code signals with corresponding replica carrier and code signals generated within the receiver to recover code measurements and carrier measurements.
For illustration, a receiver measures pseudorange, i.e., range from an antenna to an SV, by measuring phase shift between the GPS code signals and the receiver replica code signals. Such phase shift is representative of transit time and therefore a pseudorange measurement.
For attitude determination, multiple antenna/receiver sets with the antennas at fixed vehicle body locations are used. Then, differential carrier phase measurements can be made for multiple antennas with respect to a single SV. This differential measurement process eliminates the time effect (attributable to the SV), so that only the relative position effect (attributable to the multiple antennas) remains. For example, by using carrier phase measurements of the GPS signal received from an SV at two antenna/receiver sets, a differential carrier phase measurement representative of the relative position of one of the antennas with respect to the other antenna can be made.
With the use of differential measurements, measuring and processing of the relative position measurements for GPS signals received at three or more non-collinear antennas for at least two GPS SV's are used for the generation of attitude vectors required for providing full three dimensional attitude determination. For a vehicle having three antennas at fixed locations, the attitude determination using such differential carrier phase measurements represents the attitude of a plane define by the antennas.
In typical GPS attitude determination systems, such as aircraft, four antenna/receiver sets are utilized for redundancy purposes. Three of the four antennas of the antenna/receiver sets are used to define the plane for which attitude determination can be performed. Further, these antenna/receiver sets typically track more than five SVs which further provides redundancy to the computation of attitude.
Multipath reflections are the major error contributor to GPS attitude determination systems and differential position determination systems. Multipath can decrease the accuracy of such systems by an order of magnitude or more. Multipath errors in the computation of attitude occur as a result of such multipath errors being present in one or more of the carrier phase measurements utilized in such computations. Multipath errors arise because the direct path GPS signal arrival at the GPS antenna/receiver set is corrupted by associated multipath signals that arrive slightly later after reflecting from nearby reflecting surfaces. One significant source of multipath reflections, for example, is the location of reflecting surfaces near or below the horizon view of the GPS antenna.
The removal of multipath errors in the computation of positions using GPS is necessary to increase the accuracy of GPS position determination systems, including attitude determination systems. The present invention addresses the multipath problem and others as will be readily apparent to one skilled in the art from the description of the present invention as set forth in detail below.